It was not until the night of the 16th, after ZIETEN's and PIRCH's _Corps d'Armée_ had retired to Tilly and Gentinnes, that it was decided the Prussian Army should retreat upon Wavre. This decision was communicated in the Orders then transmitted from the Prussian Head Quarters to the First and Second _Corps d'Armée_ (ZIETEN's and PIRCH's) directing them to bivouac at Bierge and St Anne, in the vicinity of Wavre; as also in the Orders forwarded, on the next morning, to the bivouacs of the Third and Fourth Corps (THIELEMANN's and BÜLOW's), at Gembloux and Basse Bodecée, directing them to fall back, and bivouac at La Bavette and Dion le Mont near Wavre. ZIETEN's and PIRCH's Corps retired by Mont St Guibert, in rear of which Defile the latter Corps remained a considerable time as Rear Guard, while the former marched on to WAVRE, where it arrived about midday, crossed the Dyle, and took up its position at Bierge. PIRCH followed the same route, but took post on the right bank of the Dyle, between St Anne and Aisemont. With the first glimmering of daylight the troops, which, under the command of General JAGOW, had continued in full possession of Bry and its immediate vicinity during the night, began to retire, firstly, in the direction of Sombref, and thence to Gembloux, which they reached before the arrival of THIELEMANN's Corps. After the receipt of the Order pointing out the direction of the retreat, JAGOW conducted these troops, in the course of the 17th, towards their respective Brigades. Lieutenant Colonel SOHR, whose Cavalry Brigade with half a Horse Battery, formed the Rear Guard of the line of retreat of ZIETEN's and PIRCH's Corps, received Orders to take up a concealed position between Tilly and Gentinnes, thence to watch the movements of the Enemy; and, as soon as he found himself pressed by the latter, to fall back upon the Defile of Mont St Guibert. THIELEMANN, who, it will be recollected, had received a message from GNEISENAU, leaving it optional with him to retire by Tilly or Gembloux, according to circumstances, decided on falling back upon the latter point; being well aware that the Enemy was in possession of the Villages of St Amand and Ligny, and of the Field of Battle to within a very short distance from Sombref.